1. Field of the Invention
This invention generally relates to display driver circuits for electro-optic displays, and more particularly relates to circuits and methods for reducing the re-emission of absorbed light, for example to increase the colour gamut of organic light emitting diode displays.
2. Related Technology
Organic light emitting diodes (OLEDs) comprise a particularly advantageous form of electro-optic display. They are bright, colourful, fast-switching, provide a wide viewing angle and are easy and cheap to fabricate on a variety of substrates. Organic LEDs may be fabricated using either polymer or small molecules in a range of colours (or in multi-coloured displays), depending upon the materials used. Examples of polymer-based organic LEDs are described in WO 90/13148, WO 95/06400 and WO 99/48160; examples of so called small molecule based devices are described in U.S. Pat. No. 4,539,507.
A base structure 100 of a typical organic LED is shown in FIG. 1a. A glass or plastic substrate 102 supports a transparent anode layer 104 comprising, for example, indium tin oxide (ITO) on which is deposited a hole transport layer 106, an electroluminescent layer 108, and a cathode 110. The electroluminescent layer 108 may comprise, for example, a PPV (poly(p-phenylenevinylene)) and the hole transport layer 106, which helps match the hole energy levels of the anode layer 104 and electroluminescent layer 108, may comprise, for example, PEDOT:PSS (polystyrene-sulphonate-dope polyethylene-dioxythiopene). Cathode layer 110 typically comprises a low work function metal such as calcium and may included an additional layer immediately adjacent electroluminescent layer 108, such as a layer of aluminum, for improved electron energy level matching. Contact wires 114 and 116 and to the anode the cathode respectively provide a connection to a power source 118. The same base structure may also be employed for small molecule devices.
Other examples of materials which may be employed for layer 108 include poly(2-methoxy-5-(2′-ethyl)hexloxyphenylene-vinylene)(“MEH-PPV”), a PPV derivative (e.g. a di-alkoxy or di-alkyl derivative), a polyfluorene and/or a co-polymer incorporating polyfluorene segments, PPVs and/or related co-polymers, poly(2,7-(9,9-di-n-octyfluorene)-(1,4-phenylene-((4-secbutylphenyl)imino)-1,4-phenylene)) (“TFB”), (“PFB”) poly(2,7-(9,9-di-n-octyfluorene)-(1,4-phenylene-((4-methylphenyl)imino)-1,4-phenylene-((4-methylphenyl)imino)-1,4-phenylene))(“PFM”), poly(2,7-(9,9-di-n-octyfluorene)-(1,4-phenylene-((4-methoxyphenyl)imino)-1,4-phenylene-((4-methoxyphenyl)imino-1,4-phenylene)) (“PFMO”), poly(2,7-(9,9-di-n-octyfluorene) (“F8”) or poly (2,7-(9,9-di-n-octyfluorene)-3,6-Benzothiadiazole) (“F8BT”). Alternatively a so-called small molecule such as tris-(8-hydroxyquinoline aluminum)(“Alq3”) as described in U.S. Pat. No. 4,539,507, may be employed.
In the example shown in FIG. 1a light 120 is emitted through transparent anode 104 and substrate 102 and such devices are referred to as “bottom emitters”. Devices which emit through the cathode may also be constructed, for by keeping the thickness of cathode layer 110 less than around 50-100 nm so that the cathode is substantially transparent.
Organic LEDs may be deposited on a substrate in a matrix of pixels to form a single or multi-colour pixelated display. A multicoloured display may be constructed using groups of red, green, and blue emitting pixels. In such displays the individual elements are generally addressed by activating row (or column) lines to select the pixels, and rows (or columns) of pixels are written to, to create a display. So-called active matrix displays have a memory element, typically a storage capacitor and a transistor, associated with each pixel whilst passive matrix displays have no such memory element and instead are repetitively scanned, somewhat similarly to a TV picture, to give the impression of a steady image.
FIG. 1b shows a cross section through a passive matrix OLED display 150 in which like elements to those of FIG. 1a are indicated by like reference numerals. In the passive matrix display 150 the electroluminescent layer 108 comprises a plurality of pixels 152 and the cathode layer 110 comprises a plurality of mutually electrically insulated conductive lines 154, running into the page in FIG. 1b, each with an associated contact 156. Likewise the ITO anode layer 104 also comprises a plurality of anode lines 158, of which only one is shown in FIG. 1b, running at right angles to the cathode lines. Contacts (not shown in FIG. 1b) are also provided for each anode line. An electroluminescent pixel 152 at the intersection of a cathode line and anode line may be addressed by applying a voltage between the relevant anode and cathode lines.
Referring now to FIG. 2a, this shows, conceptually, a driving arrangement for a passive matrix OLED display 150 of the type shown in FIG. 1b, A plurality of constant current generators 200 are provided, each connected to a supply line 202 and to one of a plurality of column lines 204, of which for clarity only one is shown. A plurality of row lines 206 (of which only one is shown) is also provided and each of these may be selectively connected to a ground line 208 by a switched connection 210. As shown, with a positive supply voltage on line 202, column line; 204 comprise anode connections 158 and row lines 206 comprise cathode connections 154, although the connections would be reversed if the power supply line 202 was negative and with respect to ground line 208.
As illustrated pixel 212 of the display has power applied to it and is therefore illuminated. To create an image connection 210 for a row is maintained as each of the column lines is activated in turn until the complete row has been addressed, and then the next row is selected and the process repeated. Alternatively a row may be selected and an the column written in parallel, that is a row selected and a current driven onto each of the column lines simultaneously, to simultaneously illuminate each pixel in a row at its desired brightness. Although this latter arrangement requires more column drive circuitry it is preferred because it allows a more rapid refresh of each pixel. In a further alternative arrangement each pixel in a column may be addressed in turn before the next column is addressed, although this is not preferred because of the effect, inter alia, of column capacitance as discussed below. It will be appreciated that in the arrangement of FIG. 2a the functions of the column driver circuitry and row driver circuitry may be exchanged.
It is usual to provide a current-controlled rather than a voltage-controlled drive to an OLED because the brightness of an OLED is determined by the current flowing through it, this determining the number of photons it outputs in a voltage-controlled configuration the brightness can vary across the area of a display and with time, temperature, and age, making it difficult to predict how bright a pixel will appear when driven by a given voltage. In a colour display the accuracy of colour representations may also be affected.
FIGS. 2b to 2d illustrate, respectively, the current drive 220 applied to a pixel, the voltage 222 across the pixel, and the light output 224 from the pixel over time 226 as the pixel is address. The row containing the pixel is addressed and at the time indicated by dashed line 228 the current is driven onto the column line for the pixel. The column line (and pixel) has an associated capacitance and thus the voltage gradually rises to a maximum 230. The pixel does not begin to emit light until a point 232 is reached where the voltage across the pixel is greater than the OLED diode voltage drop. Similarly when the drive current is turned off at time 234 the voltage and light output gradually decay as the column capacitance discharges. Where the pixels in a row are all written simultaneously, that is where the columns are driven in parallel, the time interval between times 228 and 234 corresponds to a line scan period.
It is desirable to be able to provide a grayscale-type display, that is one in which the apparent brightness of individual pixels may be varied rather than simply set either on or off. In the context of this invention “grayscale” refers to such a variable brightness display, whether a pixel is white or coloured.
The conventional method of varying pixel brightness is to vary pixel on-time using Pulse Width Modulation (PWM). In the context of FIG. 2b above the apparent pixel brightness may be varied by varying the percentage of the interval between times 228 and 234 for which drive current is applied. In a PWM scheme a pixel is either full on or completely off but the apparent brightness of a pixel varies because of integration within the observer's eye.
Pulse Width Modulation schemes provide a good lines brightness response but to overcome effects related to the delayed pixel turn-on they generally employ a pre-charge current pulse (not shown in FIG. 2b) on the leading edge 236 of the driving current waveform, and sometimes a discharge pulse on the trailing edge 238 of the waveform. As a result, charging (and discharging) the column capacitance can account for half the total power consumption in displays incorporating this type of brightness control. Other significant factors which the applicant has identified as contributing to the power consumption of a display plus driver combination include dissipation within the OLED itself (a function of OLED efficiency), resistive losses in the row and column lines and, importantly in a practical circuit, the effects of a limited current driver compliance, as explained in more detail later.
FIG. 3 shows a schematic diagram 300 of a generic driver circuit for a passive matrix OLED display. The OLED display is indicated by dashed line 302 and comprises a plurality n of row lines 304 each with a corresponding row electrode contact 306 and a plurality m of column lines 308 with a corresponding plurality of column electrode contacts 310. An OLED is connected between each pair of row and column lines with, in the illustrated arrangement, its anode connected to the column line. A y-driver 314 drives the column lines 308 with a constant current and an x-driver 316 drives the row lines 304, selectively connecting the row lines to ground. The y-driver 314 and x-driver 316 are typically both under the control of a processor 318. A power supply 320 provides power to the circuitry and, in particular, to y-driver 314.
FIG. 4 shows a typical active matrix OLED driver circuit 400. A circuit 400 is provided for each pixel of the display and ground 402, Vss 404, row select 414 and column data 416 busbars are provided interconnecting the pixels. Thus each pixel has a power and ground connection and each row of pixels has a common row select line 414 and each column of pixels has a common data line 416.
Each pixel has an organic LED 406 connected in series with a driver transistor 408 between ground and power lines 402 and 404. A connection 409 of driver transistor 408 is coupled to a storage capacitor 410 and a control 412 couples gate 409 to column data line 416 under control of row select line 414. Transistor 412 is a field effect transistor (FET) switch which connects column data line 416 to gate 409 and capacitor 410 when row select line 414 is activated. Thus when switch 412 is on a voltage on column data line 416 can be stored on a capacitor 410. This voltage is retained on the capacitor for at least the frame refresh period because of the relatively high impedances of the gate connection to driver transistor 408 and of switch transistor 412 in its “off” state.
Driver transistor 408 is typically an FET transistor and passes a (drain-source) current which is dependent upon the transistor's gate voltage less a threshold voltage. Thus the voltage at gate node 409 controls the current through OLED 406 and hence the brightness of the OLED.
A voltage-driven active matrix display is described in U.S. Pat. No. 5,684,365 and a current-driven active matrix display is described in WO 99/65012. Other specific examples of OLED display drivers are described in U.S. Pat. No. 6,014,119, U.S. Pat. No. 6,201,520, U.S. Pat. No. 6,332,661, EP 1,079,361A and EP 1,091,339A; OLED display driver integrated circuits are also sold by Clare Micronix of Clare, Inc., Beverly, Mass., USA. The Clare Micronix drivers provide a current controlled drive and achieve grayscaling a conventional PWM approach; U.S. Pat. No. 6,014,119 describes a driver circuit in which pulse width modulation is used to control brightness, U.S. Pat. No. 6,201,520 describes driver circuitry in which each column driver has a constant current generator to provide digital (on/off) pixel control; U.S. Pat. No. 6,332,661 describes pixel driver circuitry it which a reference current generator sets the current output of a constant current driver for a plurality of columns, but this arrangement is not suitable for variable brightness displays; and EP 1,079,361A and EP 1,091,339A both describe similar drivers for organic electroluminescent display elements in which a voltage drive rather than a current drive is employed.
Display technologies based upon inherently emissive devices, unlike, for example, LCDs, tend to have a bright and visually pleasing appearance. There is a continuing need to improve the visual contrast of emissive displays and OLED-based displays in particular, but it is not always clear what effects contribute to contrast reduction. The applicant has recognised that the electroluminescent material normally used in both organic and inorganic light emitting diodes are generally also photoluminescent, and that this photoluminescence can contribute to contrast reduction.
Photoluminescence is a phenomenon in which, broadly speaking, a material absorbs light at one wavelength and re-emits light at a longer wavelength. This photoluminescence can be difficult to observe, even under laboratory conditions, but has the effect of giving a display a less lively appearance, particularly under bright ambient light conditions, and especially outdoors in sunlight. The applicant has found that such contrast-reducing photoluminescence can be stimulated either by absorbed ambient light or by self-absorption particularly, for example, in a display comprising a plurality of pixels, where light from one pixel can cause a neighbouring nominally off pixel to photoluminesce. In a colour display this effect can also cause a colour shift, as described further later.
In more detail, referring to FIGS. 1a and 1b, incident ambient light passes through the substrate 102, transparent anode 104, and hole transport layer 106 to the layer of electroluminescent material 108 where it is absorbed generating excitons, that is bond electron-hole pairs. Alternatively excitons may be generated by light from nearby illuminated pixels propagating through the electroluminescent layer 108, and/or transparent anode layer 104, and/or hole transport layer 106, and/or substrate 102.
With no applied field a significant fraction of these optically excited excitons rapidly radiatively decay emitting light substantially isotropically according to the photoluminescence spectra of the material or materials forming layer 108. The fraction of the excitons decaying radiatively depends upon the photoluminescence efficiency of the material and upon the applied field. When diode formed by the device is in an off state—typically, but not necessarily, when the anode and the cathode are at the same electrical potential—layer 108 is in a quiescent photo-emitting state. Thus when the display is viewed an observer sees a combination of the emitted photoluminescence and reflected and/or scattered light from the display, both of which tend to reduce the display contrast.
Prior art contrast improving techniques have concentrated upon the use of anti-reflection devices, such as filters, the circular polariser described in U.S. Pat. No. 6,211,613 (WO97/38452) assigned to the present applicant, and the black anti-reflection cathode described in U.S. Pat. No. 5,049,780. However these techniques can be insufficient, for example reducing the desired light emission. Moreover these techniques are unable to reduce the level of self-stimulated photoluminescence.
Background prior art relating the improvement of colour purity in electroluminescent display is described in EP 1 087 444, which relates to separate red, green and blue gamma correction, and in EP 1 093 322, which relates to OLED device instruction.
The applicant has recognised that contrast in a light emitting diode-based display, such as a passive or active matrix OLED-based display, may be increased by reducing the contrast-reducing photoluminescence. Where the display comprises light emitting diodes, especially organic LEDs, this photoluminescence may be reduce or quenched by reverse biasing selected ones of the light emitting diodes, that is those LEDs not emitting at any particular moment in time.
The possibility of improving OLED display contrast by reducing or quenching photoluminescence has never previously been recognised. Schemes for applying reverse bias to OLED display are known in the prior art, but these are not intended or suitable for improving contrast by the reduction of photoluminescence. Consequently these prior at reverse biasing schemes exhibit some differences from those described below for contrast-improving photoluminescence reduction.
U. Lemmer et. al., Synthetic Metals, 67 (1994) 169-172 describes the experimental observation of the basic phenomenon of photoluminescence quenching in an ITO/PPV/A1 structure.
WO98/41065 discloses the application of either polarity of driving voltage to an electroluminescent polymer-based display to drive either red light emission from an interface of the polymer or green light emission from the bulk of the polymer.
However, in both cases, the light emitting semiconductor is forward biased (the device effectively comprises two back-to-back diodes).
U.S. Pat. No. 6,201,520 describes the use of reverse biasing for non-selected pixels in a pixilated OLED display to prevent crosstalk which could otherwise be caused by the (electrically) semi-excited state of the non-selected pixels. However U.S. Pat. No. '520 does not specify any particular value of reverse bias drive and does not provide any teaching on the application of a reverse bias drive sufficient to provide an improved contrast display by photoluminescence quenching. Furthermore the mechanism for applying a reverse bias in U.S. Pat. No. '520 limits the reverse bias voltage to the forward bias voltage whereas, generally speaking, it is preferable to apply a larger reverse bias voltage than the forward voltage to achieve adequate photoluminescence reduction for improved contrast.
U.S. Pat. No. 5,965,901, assigned to the present applicant, describes the use of a pulse driving scheme for an organic light-emitting polymer device to improve device lifetime in which positive pulses are separated by negative (reverse bias) pulses. However this document does not contemplate applying reverse bias to some pixels at the same time as applying forward bias to others and is thus unsuitable for reducing photoluminescence stimulated by emission from pixels within the display. Furthermore again the document does not provide any teaching on the application of a reverse bias drive sufficient to provide an improved contrast display by photoluminescence quenching.
EP 1094438A describes the periodic application (for example, every frame) of reverse bias to reduce leakage current due to through-film shorts.